We have previously characterized mouse and human cDNA clones encoding a new transmembrane EGF-like protein. The predicted protein from this cDNA (termed delta-like or dlk) is highly homologous to invertebrate homeotic transmembrane proteins, including Notch and Delta of Drosophila. These proteins also contain EGF-like repeats at their extracellular domains which intervene in receptor-ligand interactions between cells. These interactions generate signals which are involved in differentiation decisions. The function of dlk is unknown. A recent report identified Pref-1 as a new EGF-like molecule able to inhibit the adipocyte differentiation of 3T3L1 cells. Studies in our laboratory have confirmed that Pref-1 is, in fact, a polymorphic variant of dlk. Other polymorphisms, as well as alternately spliced dlk species, have been identified. The 3T3L1 model is currently being used to study the domains of dlk that are needed to block 3T3L1 differentiation. The homology with Drosophila Delta suggests that dlk could act as a ligand for TAN-1, the human Notch homolog, which was identified by its putative role in human leukemogenesis. Potential cross-talk interactions between dlk and TAN-1 are being studied using 3T3L1 cells as a model. Preliminary data indicate that 3T3L1 cells with diminished expression of TAN-1 are also unable to differentiate to adipocytes. Expression of alternately spliced TAN-1 mRNA has also been identified by RT-PCR and RNAse protection assays. The putative proteins expressed by the spliced RNAs contain internal deletions of the ankyrin repeat intracellular domain of TAN-1 which is important for TAN-1 function. Production of rabbit antisera against different domains of dlk and TAN-1 proteins is ongoing. Studies of ligand binding and potential interactions between dlk and TAN-1 will be the focus of future investigations.